1. Field of the Invention
The present invention relates to a magnetic resonance imaging (MRI) apparatus, and particularly to a high field magnetic resonance imaging apparatus and a method for obtaining a high signal-to-noise ratio by a receiving coil thereof.
2. Description of the Prior Art
Magnetic resonance imaging is an imaging technology for obtaining the internal structural information of an object by means of the magnetic resonance phenomena. MRI is broadly used in the field of medical imaging due to its significant advantages, such as a large number of imaging parameters, high resolution, tomography in any layer required, non-destructive imaging, etc.
In a middle or low field MRI apparatus (basic magnetic field=0.5T), the direction of the basic magnetic field is perpendicular to the patient's limb in a receiving coil. A radio frequency (RF) receiving coil is usually a loop (i.e. loop type) coil, such as a single loop coil shown in FIG. 1, or a solenoid coil shown in FIG. 2 or a Helmholtz coil pair, etc. In a high field MRI apparatus (basic magnetic field=1.5T), since the patient's limb is parallel to the horizontal direction of the basic magnetic field, the RF receiving coil can only have a cage type or a surface loop coil structure, and the receiving coil of the surface loop coil structure is shown in FIG. 3. However, in practical applications, the signal-to-noise ratio (SNR) achievable by an RF receiving coil of the cage type or the surface loop structure is lower than that of a loop type RF receiving coil.
FIG. 4 shows a schematic view of the example of a wrist coil used in a high field MRI apparatus in the prior art. FIG. 5 shows a case of the application of the magnetic resonance imaging to a patient's wrist in the prior art, wherein the patient is lying on a bed 503 with the wrist to be scanned in a receiving coil 502, and the receiving coil 502 is positioned beside the patient's body and within the magnetic field generated by a basic magnet 501. FIG. 6 shows another case of the application of the magnetic resonance imaging to a patient's wrist in the prior art, wherein the patient is lying on a bed 603 with the wrist to be scanned in a receiving coil 602, and the receiving coil 602 is positioned beyond the patient's head and within the magnetic field generated by a basic magnet 601.
It can be clearly seen from FIGS. 5 and 6 that the patient's limb is parallel to the horizontal direction of the basic magnetic field, so the wrist coil can only have a surface loop coil, so its SNR is lower than that of a loop type wrist coil.
Moreover, in the case as shown in FIG. 5, since the patient's limb is placed flat beside his or her body, the wrist coil is nearly beyond the field of view (abbreviated as FOV) of the MRI apparatus. In the case as shown in FIG. 6, the receiving coil 602 and the patient's wrist are positioned beyond and to one side of the patient's head. Although the location of the receiving coil 602 in the case as shown in FIG. 6 is slightly better than that of the receiving coil 502 shown in FIG. 5, it is still not ideal, and the patient in this position is not comfortable, therefore it is not suitable to keep this position for a long time.
Generally, the MRI apparatus and method in the prior art have the following disadvantages:
1) due to the differences in the relationships of the patient's limb with the direction of the basic magnetic field and the manners of positioning the receiving coil between a middle or low field MRI and a high field MRI, a loop type receiving coil cannot be used in an current high field MRI apparatus;
2) the receiving coil for a high field MRI apparatus can only be of a case type or a surface loop structure, which cannot achieve a high SNR;
3) the specific position of a wrist coil is nearly beyond the FOV of the MRI apparatus; and
4) the specific position of the wrist coil affects the comfort of a patient when having an MRI operation.